This proposal is designed to assess the influence of cardiac drugs on coronary vascular resistance, oxygen consumption, myocardial contractility, and myocardial efficiency. These actions are to be assessed in a special Starling type preparation in which the pressure gradient for coronary flow is controlled by a separate reservoir system from that which regulates left ventricular afterload. This preparation allows us to measure, on a beat-to-beat basis, total coronary blood flow and to fractionate this flow into its systolic and diastolic components. Afterload, heart rate, and systolic comprehesion factors are maintained constant. The latter is achieved in a pumping heart by using a reservoir system and servo-operated pump connected to the left ventricular chamber. Left ventricular wall tension is measured with a modified Walton-Brodie strain gauge. An on-line digital computer processes the data, with samples obtained at approximately 8 second intervals, producing immediate readouts of afterload, heart rate, left ventricular systolic and diastolic tensions, systolic and diastolic coronary blood flows, systolic and diastolic time intervals, cardiac work, and various indices of cardiac performnce such as dp/dt and time to peak ventricular pressure. Continuous on-line oxygen consumption is also recorded. Drugs that directly influence the coronary vascular as well as common inotropic agents will be studied. Distribution of flow across the myocardial wall will be studied using microspheres and nutritive vs. non-nutritive flow and will be assessed using 86Rb as an indicator.